1. Field of the Invention
Electrohydraulic actuators are generally constituted by a servo-valve, i.e. a hydraulic slide valve with electrically controlled slide.
2. History of the Related Art
Such actuators are in particular intended for controlling a hydraulic single- or double-acting jack and are provided to cause any mechanism to operate in reciprocating motion. The actuators are often equipped with an adjustable mechanical device defining a direction of action of the actuator-jack assembly upon the disappearance of the control voltage of the electrical element for controlling the actuator.
In that case, i.e. when the lack of current brings about a defective servo-control, three possibilities offer themselves:
tendency to open; PA1 tendency to close; PA1 zero tendency, i.e. the actuator and its jack remain in the same position.
Taking into account the drifts, the latter state is not conserved in time, the tendency becomes virtually random, which is incompatible in certain applications such as the regulation of the speed of hydraulic turbines. It is generally necessary in that case, on the one hand, to memorize and conserve the position of the electro-hydraulically controlled jack in the event of breakdown of electrical energy and, on the other hand, to be able to control manually the position of the jack without electrical energy.
Solutions for solving this problem have already been proposed, but they result in complicated, therefore expensive assemblies which are of reduced reliability.